An electric double-layer capacitor includes a capacitor element, a metal case, and a rubber sealing member. The capacitor element includes a positive electrode provided with an electrode layer including activated carbon on a metal foil, a negative electrode having the same configuration as that of the positive electrode, and a separator interposed between the positive electrode and the negative electrode. The capacitor element is accommodated in the metal case together with an electrolyte, and an opening of the metal case is sealed with the rubber sealing member. Then, in order to lead out electrodes from the capacitor element, a connecting member such as a lead wire is joined to each of the positive electrode and the negative electrode, and the electrodes are led out through the above-mentioned rubber sealing member.
An electrolyte of a conventional electric double-layer capacitor employs quaternary ammonium salt or quaternary phosphonium salt as a solute. These solutes are chemically active characteristics. Therefore, when an electric double-layer capacitor using these solutes is charged and discharged repeatedly in severe conditions such as, particularly, high temperatures, pH of the electrolyte in a vicinity of the negative electrode is biased to an alkaline side, the electrolyte may creep on the surface of the negative electrode and may leak out from the rubber sealing member to the outside. Furthermore, the rubber sealing member and the lead wire for extracting the negative electrode may corrode, because they are brought into contact with the electrolyte creepage.
A proposal for coping with such problems is described with reference to FIG. 3. FIG. 3 is a schematic view showing a current collector and a rubber sealing member used in a conventional electric double-layer capacitor. Tab terminal 102 as a lead wire is connected to current collector 101 constituting a negative electrode, and tab terminal 102 is inserted into through-hole 103A provided in rubber sealing member 103.
In such a configuration, in order to enhance resistance to corrosiveness due to liquid leakage, chemical conversion coating is formed on surfaces of current collector 101 and tab terminal 102 using oxidizing chemicals. Then, rubber sealing member 103 having through-hole 103A is subjected to resin vulcanization or peroxide vulcanization with butyl rubber, so that hardness of rubber sealing member 103 is made to be a range from 60 to 90 (according to Wallace hardness meter). Such a method has been proposed.
Thus, even if quaternary ammonium salt or quaternary phosphonium salt is used as a solute, leakage of the electrolyte from rubber sealing member 103 to the outside can be suppressed (see, for example, Patent Literature 1).